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Simultaneously opening Opening a file multiple times that is already open has implementation-defined behavior. On some platforms, this is not allowed. On others, it might result in race conditions.

Non-Compliant Coding Example

, according to the C Standard, 7.21.3, paragraph 8 [ISO/IEC 9899:2011]:

Functions that open additional (nontemporary) files require a file name, which is a string. The rules for composing valid file names are implementation-defined. Whether the same file can be simultaneously open multiple times is also implementation-defined.

Some implementations do not allow multiple copies of the same file to be open at the same time. Consequently, portable code cannot depend on what will happen if this rule is violated. Even on implementations that do not outright fail to open an already-opened file, a TOCTOU (time-of-check, time-of-use) race condition exists in which the second open could operate on a different file from the first due to the file being moved or deleted (see FIO45-C. Avoid TOCTOU race conditions while accessing files for more details on TOCTOU race conditions).

Noncompliant Code Example

This noncompliant The following non-compliant code example logs the program's state at runtime.:

#include <stdio.h>
 
void do_stuff(void) {
   FILE *logfile = fopen("log", "a");
  if (logfile == NULL) {
    /* Check for errors, write Handle error */
  }

  /* Write logs pertaining to do_stuff(), etc. */
  fprintf(logfile, "do_stuff\n");
}

int main(void)
 {
   FILE *logfile = fopen("log", "a");
  if (logfile == NULL) {
    /* Check for errors, write Handle error */
  }

  /* Write logs pertaining to main(), etc. */
  fprintf(logfile, "main\n");

  do_stuff();
 
  if (fclose(logfile) == EOF) {
    /* Handle ...error */
  }
  return 0;
}

Because However, the file log is opened twice simultaneously. The result is (once in main() and again in do_stuff()), this program has implementation-defined and potentially dangerousbehavior.

Compliant Solution

In this compliant solution, a reference to the file pointer is passed around so that the file does not have to be opened twice separatelyas an argument to functions that need to perform operations on that file. This reference eliminates the need to open the same file multiple times.

#include <stdio.h>
 
void do_stuff(FILE **filelogfile) {
  FILE *logfile = *file;

  /* Check for errors, writeWrite logs pertaining to do_stuff,() etc. */
  fprintf(logfile, "do_stuff\n");
}

int main(void) {
  FILE *logfile = fopen("log", "a");
  if (logfile == NULL) {
    /* Handle error * Check for errors, write/
  }

  /* Write logs pertaining to main,() etc. */
  fprintf(logfile, "main\n");

  do_stuff(&logfile);
 
  if (fclose(logfile) == EOF) {
    /* Handle ...error */
  }
  return 0;
}

Risk Assessment

Simultaneously opening a file multiple times could can result in abnormal program termination or a data integrity violationunexpected errors and nonportable behavior.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19.3, "Files"

Automated Detection

Related Guidelines

 Bibliography

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Image Added Image Added Image AddedImage Removed      09. Input Output (FIO)       FIO32-C. Detect and handle file operation errors